1. Field of the Invention
This invention relates to a tobacco filter stock. More particularly, it relates to a tobacco filter stock formed using a tow of crystalline polyolefin filaments, and a process for producing a tobacco filter which comprises applying the stock to a tobacco filter paper winder to mold the stock into a tobacco filter without use of any adhesion means such as adhesives, hot-melt adhesion, etc.
2. Description of the Related Art
As the stock for tobacco filters, cellulose triacetate filaments have been generally used, and paper winders themselves for tobacco filter-producing machines have generally been unified into those for cellulose triacetate filaments. On the other hand, as a filament stock in place of cellulose triacetate filaments, polyolefin filaments, particularly polypropylene filaments, have been proposed mainly because of cost, and have been practically used in part (Japanese patent application laid open No. Sho 61-247368/1986, U.S. Pat. No. 4,546,040, Japanese patent publication No. Sho 55-402315/1980, etc.). As an example of the process for producing a tobacco filter using polypropylene filaments, a tow having a total denier of about 40,000 to 60,000 is passed through a filaments opening machine using air or steam to subject it to opening treatment; the bundling of the tow is improved by adding a suitable binder or hot-melt adhesion; the resulting tow is fed, onto a paper winder as a tobacco filter-producing machine; a paper is wound around the tow while molding the tow into a rod form and solidifying the resulting material to obtain a product (Japanese patent application laid-open No. Sho 61-247368/1986). This process however, has raised various problems such as the necessity of filament binders and an adhesion step environmental pollution due to solvents of the binders, inferior processability, high cost, etc. Further, U.S. Pat. No. 4,546,040 discloses a process for producing a tobacco filter stock by stretching a sliver consisting of 3,000 to 10,000 multifilaments of polyolefin conjugate fibers having three-dimensional crimps (5 to 7 crimps/cm) at a temperature of 15.degree. to 70.degree. C. During the above-mentioned stretching of the fibers, fine voids and micropores are formed. The sliver is annealed at 70.degree. to 140.degree. C., followed by winding a paper thereon by means of a filter paper winder and cutting the resulting material to a predetermined length to obtain a product. However, the tobacco filter produced using such a sliver of polyolefin conjugate fibers having three-dimensional crimps has the so-called cavities (vacant parts) in spite of a heavy basis weight (weight per unit cross-section area), an insufficient hardness and an inferior whiteness; hence only a product having a considerably low commercial value can be produced.
Further, the above-mentioned Japanese patent publication No. Sho 55-402315/1980 also discloses a process for producing a tobacco filter by subjecting two or more kinds of components having different melting points to conjugate spinning to prepare polyolefin filaments having a latent crimp, and subjecting the filaments to heat-treatment at a temperature between the melting points of both the components to subject these filaments to self-adhesion by hot-melt adhesion to one another thereby stabilizing the resulting filament bundle. However, because the process requires a treatment step of hot-melt adhesion, the resulting tow has a heavy basis weight like the above-mentioned one and the production rate is very low.
Further, heretofore when a tobacco filter or the like has been produced using polyolefin filaments in 100% by weight without any adhesive, the resulting product has had a lower Young's modulus than that of cellulose triacetate filaments. Thus, there have been drawbacks in that it is inferior as to the hardness of the tobacco filter. Further, a number of falling filaments due to non-adhesion and cavities in the tobacco filter are likely to occur, and the resulting filter has a very uneven whiteness.